1. Field of the Invention
The present invention relates to a wafer carrier, and more particularly, to a wafer carrier adapted for use in a double-sided process.
2. Description of the Prior Art
Very large scale integration (VLSI) fabrication is based on a semiconductor wafer, and implemented with tens of, or even hundreds of, semiconductor processes so as to form a plurality of dies having devices and connections therein. These dies are then segmented and packaged to form a plurality of chips for different applications. Therefore, the wafer has to be transferred among reaction chambers of different apparatuses for undergoing different semiconductor processes, such as thin film deposition process, lithography process, etching process, polishing process, etc.
Normally, the wafer is sucked by a vacuum chuck, and transferred among different apparatuses. Please refer to FIG. 1, which is a schematic diagram of a wafer 10 carried by a vacuum chuck 12. As shown in FIG. 1, the wafer 10 is sucked by the vacuum chuck 12 for being loaded in or out of different apparatuses for performing different processes. The vacuum chuck 12 has a plurality of openings 14 communicating with each other, and a pump (not shown). The openings 14 are vacuumed with the pump (not shown) so as to suck the wafer 10 to be positioned on the surface of the vacuum chuck 12.
While the wafer 10 undergoes single-sided processes, the vacuum chuck 12 is able to provide a good fastening effect. However, as the design of semiconductor devices becomes complicated, more and more devices or structures, such as print heads, chip scale packages, MEMS structures, have to be formed by double-sided processes. In such a case, when the front side pattern is formed, the wafer 10 has to be turned over, sucked by the vacuum chuck 12 on the front side, and undergoes back side processes. Consequently, the front side pattern may impede the fastening effect of the vacuum chuck 12, and the front side pattern may be damaged by the vacuum chuck 12 during transportation. Particularly in the case of forming MEMS devices, the wafer 10 is easier to be damaged during the transportation due to its thickness (less than 300 μm). In addition, MEMS devices frequently have through-hole structures, and thus cannot be delivered by the vacuum chuck 12.
Please refer to FIG. 2 and FIG. 3. FIG. 2 and FIG. 3 are schematic diagrams illustrating a wafer 20 carried by an electrostatic chuck (E chuck) 30, where FIG. 2 illustrates the E chuck 30 carrying the wafer 20 from the back side 24, and FIG. 3 illustrates the E chuck 30 carrying the wafer 20 from the front side 22. As shown in FIG. 2, the back side 24 of the wafer 20 is bonded to a carrier wafer 28 with a bonding layer 26, and the E chuck 30 is used to attract the bottom of the carrier wafer 28 and deliver the wafer 20. The carrier wafer 28 is normally a bare wafer, and the bonding layer 26 works to bond the wafer 20 and the carrier wafer 28. The wafer 20 is therefore delivered between each apparatus, and undergoes different processes to form a front side pattern 22A in the front side 22. As described earlier, many semiconductor devices or MEMS devices have to be completed by double-sided processes, thus the bonding layer 26 is then removed, and the wafer 20 is turned over to form a back side pattern 24A in the back side 24 of the wafer 20.
As shown in FIG. 3, when the front side pattern 22A is formed, the wafer 20 is turned over and bonded to the carrier wafer 28 with another bonding layer 32 in the front side 22. The carrier wafer 28, which carries the wafer 20, is then attracted by the E chuck 30, and delivered to different apparatuses to form the back side pattern 24A. It should be appreciated that in the process of defining the back side pattern 24A, such as a lithography process, accurate alignment is required to ensure device reliability. Normally, the wafer 20 has several alignment marks in the front side 22 so that the apparatus can accurately align the wafer 20 by using the alignment marks as a basis during the back side process. However, the conventional method of supporting the wafer 20 using the carrier wafer 28 is not allowed for performing the alignment action, and thus it is easy to cause deviation of the back side pattern 24A. As shown in FIG. 3, the back side pattern 24A of the wafer 20 deviates from the predetermined center line.
Due to the aforementioned drawbacks, a wafer carrier able to deliver the wafer stably without interfering with the alignment action is necessary.